Low-current probe card

ABSTRACT

A low-current probe card for measuring currents down to the femtoamp region includes a dielectric board, such as of glass-epoxy material, forming an opening. A plurality of probing devices, such as ceramic blades, are edge-mounted about the opening so that the probing elements or needles included thereon terminate below the opening in a pattern suitable for probing a test device. A plurality of cables are attached to the card for respectively connecting each device to a corresponding channel of a test instrument. The on-board portion of each cable is of coaxial type and includes an inner layer between the inner dielectric and outer conductor for suppressing the triboelectric effect. An inner conductive area and a conductive backplane that are respectively located below and on one side of each device are set to guard potential via the outer conductor of the corresponding cable so as to guard the signal path on the other side of the device. The lead-in portion of each cable, which is detachably connected to the corresponding on-board portion through a plug-in type connector, is of triaxial type and includes, besides the inner layer between the inner dielectric and outer conductor, a second inner dielectric and second outer conductor. A conductive cover and an outer conductive area that substantially enclose the components on the card are set to shield potential via the second outer conductor and connector.

This is a continuation of U.S. patent application Ser. No. 08/988,243filed Dec. 1, 1997.

BACKGROUND OF THE INVENTION

The present invention relates to probe cards which are used for probingtest devices, such as integrated circuits on a wafer, and, inparticular, relates to probe cards that are suitable for use inmeasuring ultra-low currents.

Typically, in the construction of a probe card, a dielectric board isused as a base. A plurality of probing devices are mounted in radialarrangement about an opening in the board so that the probing elementsof these devices, which may, for example, comprise slender conductiveneedles, terminate below the opening in a pattern suitable for probingthe contact sites of the test device. The probing devices areindividually connected to the respective channels of a test instrumentby a plurality of interconnecting lines, where the portion of each linethat extends between the corresponding probing device and the outer edgeof the dielectric board may comprise an interconnecting cable or aconductive trace pattern formed directly on the board. In oneconventional type of setup where the test devices are integratedcircuits formed on a semiconductive wafer, the probe card is mounted bymeans of a supporting rig or test head above the wafer, and a supportbeneath the wafer moves the wafer so that each device thereon isconsecutively brought into contact with the needles or probing elementsof the probe card

With particular regard to probe cards that are specially adapted for usein measuring ultra-low currents (down to the femtoamp region or lower),probe card designers have been concerned with developing techniques foreliminating or at least reducing the effects of leakage currents, whichare unwanted currents that can flow into a particular cable or channelfrom surrounding cables or channels so as to distort the currentmeasured in that particular cable or channel. For a given potentialdifference between two spaced apart conductors, the amount of leakagecurrent that will flow between them will vary depending upon the volumeresistivity of the insulating material that separates the conductors,that is, if a relatively lower-resistance insulator is used, this willresult in a relatively higher leakage current. Thus, a designer oflow-current probe cards will normally avoid the use of rubber-insulatedsingle-core wires on a glass-epoxy board since rubber and glass-epoxymaterials are known to be relatively low-resistance insulators throughwhich relatively large leakage currents can flow.

One technique that has been used for suppressing interchannel leakagecurrents is surrounding the inner core of each lead-in wire with acylindrical “guard” conductor, which conductor is maintained at the samepotential as the inner core by a feedback circuit in the output channelof the test instrument. Because the voltage potentials of the outerguard conductor and the inner conductive core are made to substantiallytrack each other, negligible leakage current will flow across the innerdielectric that separates these conductors regardless of whether theinner dielectric is made of a low- or high-resistivity material.Although leakage current can still flow between the guard conductors ofthe respective cables, this is typically not a problem because theseguard conductors, unlike the inner conductive cores, are at lowimpedance. By using this guarding technique, significant improvement maybe realized in the low-level current measuring capability of certainprobe card designs.

To further improve low-current measurement capability, probe cards havebeen constructed so as to minimize leakage currents between theindividual probing devices which mount the probing needles or otherelements. With respect to these devices, higher-resistance insulatingmaterials have been substituted for lower-resistance materials andadditional conductive surfaces have been arranged about each device inorder to perform a guarding function in relation thereto. In one type ofassembly, for example, each probing device is constructed using a thinblade of ceramic material, which is a material known to have arelatively high volume resistivity. An elongate conductive trace isprovided on one side of the blade to form the signal line and abackplane conductive surface is provided on the other side of the bladefor guarding purposes. The probing element of this device is formed by aslender conductive needle, such as of tungsten, which extends in acantilevered manner away from the signal trace. Such devices arecommercially available, for example, from Cerprobe Corporation based inTempe, Ariz. During assembly of the probe card, the ceramic blades areedge-mounted in radial arrangement about the opening in the card so thatthe needles terminate within the opening in a pattern suitable forprobing the test device. The conductive backplane on each blade isconnected to the guard conductor of the corresponding cable and also tocorresponding conductive pad or “land” adjacent the opening in the probecard. In this manner each conductive path is guarded by the backplaneconductor on the opposite side of the blade and by the conductive landbeneath it.

It has been found, however, that even with the use of guarded cables andceramic probing devices of the type just described, the level ofundesired background current is still not sufficiently reduced as tomatch the capabilities of the latest generation of commerciallyavailable test instruments, which instruments are able to monitorcurrents down to one femtoamp or less. Thus, it was evident that otherchanges in probe card design were needed in order to keep pace with thetechnology of test instrument design.

In the latest generation of probe cards, efforts have been directedtoward systematically eliminating low-resistance leakage paths withinthe probe card and toward designing-extensive and elaborate guardingstructures to surround the conductors along the signal path. Forexample, in one newer design, the entire glass-epoxy main board isreplaced with a board of ceramic material, which material, as notedabove, presents a relatively high resistance to leakage currents. Inthis same design, the lead-in wires are replaced by conductive signaltraces formed directly on the main board, which traces extend from anouter edge of the main board to respective conductive pads that surroundthe board opening. Each pad, in turn, is connected to the signal path ofa corresponding ceramic blade. In addition, a pair of guard traces areformed on either side of each signal trace so as to further isolate eachtrace against leakage currents.

In yet another of these newer designs, a main board of ceramic materialis used having three-active layers to provide three dimensionalguarding. Above this main board and connected thereto is a four-quadrantinterface board that includes further guard structures. Between thesetwo board assemblies is a third unit including a “pogo carousel.” Thispogo carousel uses pogo pins to form a plurality of signal lines thatinterconnect the interface board and the lower main board. It will berecognized that in respect to these pogo pins, the effort to replacelower resistance insulators with higher resistance insulators has beentaken to its practical limit, that is, the insulator that would normallysurround the inner conductor has been removed altogether.

The probe card designs which have just been described represent thecurrent state-of-the-art. From the foregoing examples, it will be seenthat a basic concern in the art has been the suppression of interchannelleakage currents. Using these newer designs, it is possible to measurecurrents down to nearly the femtoamp level. However, the ceramicmaterial used in these newer designs is relatively more expensive thanthe glass-epoxy material it replaces. Another problem with ceramicmaterials is that they are relatively susceptible to the absorption ofsurface contaminants such as can be deposited by the skin-duringhandling of the probe card. These contaminants can decrease the surfaceresistivity of the ceramic material to a sufficient extent as to producea substantial increase in leakage current levels. In addition, the moreextensive and elaborate guarding structures that are used in these newerdesigns has contributed to a large increase in design and assemblycosts. Based on these developments it may be anticipated that onlygradual improvements in the low-current measurement capability of thecards is likely to come about, which improvements, for example, willresult from increasingly more elaborate guarding systems or from furtherresearch in the area of high resistance insulative materials.

It should also be noted that there are other factors unrelated to designthat can influence whether or not the potential of a particular probecard for measuring low-level currents will be fully realized. Forexample, unless special care is taken in assembling the probe card, itis possible for surface contaminants, such as oils and salts from theskin or residues left by solder flux, to contaminate the surface of thecard and to degrade its performance (due to their ionic character, suchcontaminants can produce undesirable electrochemical effects).Furthermore, even assuming that the card is designed and assembledproperly, the card may not be suitably connected to the test instrumentor the instrument may not be properly calibrated so as to completelynull out, for example, the effects of voltage and current offsets. Inaddition, the probe card or the interconnecting lines can serve aspickup sites for ac fields, which ac fields can be rectified by theinput circuit of the test instrument so as to cause errors in theindicated dc values. Thus, it is necessary to employ proper shieldingprocedures in respect to the probe card, the interconnecting lines andthe test instrument in order to shield out these field disturbances. Dueto these factors, when a new probe card design is being tested, it canbe extremely difficult to isolate the causes of undesirable backgroundcurrent in the new design due to the numerous and possibly interactingfactors that may be responsible.

In view of the foregoing, what is needed is a probe card that is capableof being used for the measurement of ultra-low level currents but yetcan be inexpensively manufactured from relatively low-cost materials inaccordance with a relatively straightforward assembly process.

SUMMARY OF THE INVENTION

In accordance with the present invention, the inventor has discoveredthat the primary problem, at least at some stage in the design, is nothow best to suppress the leakage currents that flow between thedifferent signal channels but rather how best to suppress those currentsthat internally arise in each cable or signal channel as a result of thetriboelectric effect. In a guarded cable, triboelectric currents canarise between the guard conductor and the inner dielectric due tofriction therebetween which causes free electrons to rub off theconductor and creates a charge imbalance that causes current to flow.Once the inventor recognized that this triboelectric effect might be thecritical problem, he proceeded to test this insight by substituting“low-noise” cables for the guarded cables that had heretofore been used.These low-noise cables, which were custom-made in order to meet sizeconstraints, made a significant difference to the low currentmeasurement capability of the probe card. Indeed, even though thesecables were used in connection with a relatively inexpensive glass-epoxyboard, and even though, under conventional thinking, this type ofmaterial did not possess sufficiently high resistance to permitultra-low current measurements, the inventor was able to achieve currentmeasurements down to the femtoamp region. Within weeks of thisdiscovery, the commercial value of this invention became readilyapparent when measurement data taken from a prototype of the subjectprobe card was instrumental to a customer purchase order for two probingstations worth hundreds of thousands of dollars apiece.

It will be noted that the inventor does not claim to have discovered anew solution to the problem of the triboelectric effect. A relativelystraightforward solution to this problem can be found in the field ofcable technology wherein it is known how to construct a “low-noise”cable by using an additional layer of material between the outerconductor and the inner dielectric, which material is of suitablecomposition for suppressing the triboelectric effect. This layer, inparticular, includes a nonmetallic portion that is physically compatiblewith the inner dielectric so as to be prevented from rubbing excessivelyagainst this dielectric and, on the other hand, includes a portion thatis sufficiently conductive that it will immediately dissipate any chargeimbalance that may be created by free electrons that have rubbed off theouter conductor. It is not claimed by the inventor that this particularsolution to the triboelectric effect problem is his invention. Rather itis the recognition that this specific problem is a major source ofperformance degradation in the field of low-current probe card designthat the inventor regards as his discovery.

In retrospect, one can speculate as to why the significance of thetriboelectric effect was not recognized sooner by investigators in theart of probe card design. One possible reason is that verifying theimportance of this effect is not merely a matter of replacing guardedcables with low-noise cables. As indicated, in the Background sectionhereinabove, traces formed directly on the main dielectric board havelargely replaced guarded cables in the newer generation of probe carddesigns, so that in order to begin with a design where this problem isamendable to a straightforward solution, one must return to an older andpresumably less effective technology. Moreover, because of the nondesignrelated factors specified in the Background section, one of ordinaryskill who assembled and then tested a probe card that included low-noisecables would not necessarily detect the superior capability of thisprobe card for low current measurements. For example, surfacecontaminants deposited on the probe card during its assembly might raisethe background level of current to a sufficient extent that the effectof the low-noise cables is concealed. To this it may be added that thedirection taken in the art of probe card design, where the focus hasbeen on the problem of suppressing interchannel leakage currents, hasprovided solutions which, by happenstance, have also substantiallyresolved the triboelectric effect problem. These solutions, whichincluded replacing cables with trace-like conductors on ceramic boardsor using signal lines in which no insulator at all surrounds the signalconductor (as in the case of signal lines formed by pogo pins) arecomplicated and expensive compared to the inventor's relativelystraightforward solution to the triboelectric effect problem. However,the indirect and mitigating effect of these alternative solutions, whichserved to conceal the problem, does help to explain why a more directsolution to the triboelectric effect problem was overlooked even bysophisticated designers of state-of-the-art probe cards.

In accordance, then, with the present invention, a probe card isprovided for use in measuring ultra-low currents, which probe cardincludes a dielectric board, a plurality of probing devices that areedge-mounted in radial arrangement about an opening in the board, and aplurality of cables for connecting each probing device to acorresponding channel of a test instrument. These cables are of suitableconstruction to be used in a guarded mode, that is, they include anouter conductor that surrounds the inner conductor or core of the cable,which outer conductor can be used as a guard conductor in relation tothe inner conductor. Furthermore, these cables include an inner layer ofmaterial between the outer conductor and the underlying innerdielectric, which layer is of suitable composition for reducingtriboelectric current generation between the inner dielectric and theouter conductor to less than that which would occur were the innerdielectric and the outer conductor to directly adjoin each other.

In accordance with the foregoing construction, a probe card is providedin which a significant source of background current is suppressed in arelatively straightforward manner thereby eliminating the need forproviding complicated and expensive structures to suppress other lesssignificant sources in order to achieve the capability of measuringultra-low currents. This and other objectives, features and advantagesof the invention will be more readily understood upon consideration ofthe following detailed description of the invention taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a low-current probe card constructed inaccordance with the present invention where a portion of a conductivecover included on the card has been broken away to reveal elementshidden thereunder.

FIG. 2 is a side elevational view of the low-current probe card of FIG.1.

FIG. 3 is a broken-away plan view of the low-current probe card of FIG.1 in the region surrounding the opening in the card, in which view thecover has been removed to show the underlying elements.

FIG. 4 is a sectional view taken along lines 4-4 in FIG. 3.

FIG. 5 is a cross sectional view of the lead-in portion of a particularone of the cables as taken along lines 5-5 in FIG. 1.

FIG. 6 is a cross sectional view of the on-board portion of a particularone of the cables as taken along lines 6-6 in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows an exemplary low-current probe card 20 constructed inaccordance with the present invention. Referring also to FIGS. 2 and 4,this card includes a plurality of probing elements 22. In the preferredembodiment shown, these probing elements comprise tungsten needles whichextend in generally radial arrangement so that their tips terminate in apattern suitable for probing the contact sites on a test device (notshown). For example, if the device to be tested is an individual circuiton a semiconductive wafer having a square-like arrangement of contactssites, the needles would correspondingly terminate in a square-likepattern. Further included on the probe card are a plurality of cables24. During use of the probe card, each of these cables is connected to aseparate channel of a test instrument (not shown) thereby electricallyconnecting each channel to a corresponding one of the probing element.It will be recognized that although only six cables and probing elementsare shown in the drawings, this small number was selected for ease ofillustration only. Actually, twenty-four cables and probing elements areincluded with the probe card shown in the drawings, although an evengreater number than this can be provided as explained hereinbelow.

The exemplary probe card 20 includes a dielectric board 26 which, in thepreferred embodiment shown, is made of FR4 glass-epoxy material. Thisboard generally serves as the base of the probe card on which the othercard components are mounted. As shown in FIG. 1, each cable includes alead-in portion 28 and an onboard portion 30, which respective portionsare detachably connected together by means of a plug-in type connector32, thereby facilitating the separate testing of the lead-in portions ofthe cables. As further described hereinbelow, these cables includeconductive and dielectric layers in coaxial arrangement with each otherand further include at least one layer of material within each cableadapted for suppressing the triboelectric effect so as to minimize anyundesirable currents that would otherwise be generated internally ineach cable due to this effect. This layer of material together withcertain other structures included on the probe card enable the card tobe used for the measurement of ultra-low currents down to one femtoampor less. In comparison to a design which uses conductive traces on aceramic board, not only is the present design less expensive to make, italso is less susceptible to surface contaminants and can, for example,continue to operate in a satisfactory manner in low-temperature testenvironments where moisture can condense on the main board.

Referring to FIGS. 3 and 4, the on-board portion 30 of each cable iselectrically connected to one end of a probing device 34, where theother end of this device includes the probing element or needle 22. Withrespect to the exemplary probe card 20 shown, each probing deviceincludes a dielectric substrate 36 preferably formed of ceramic or acomparable high-resistance insulating material. This ceramic substrateor “blade” has a pair of broad parallel sides interconnected by a thinedge. Formed on one side of each blade is an elongate conductive path 38while the other side includes a backplane conductive surface 40. A lowerconductor (not shown) is formed along a bottom portion of each edge,which conductor electrically connects to the corresponding elongateconductive path 38. In turn, each probing element or needle 22 iselectrically connected to a respective one of these conductors so thatthe element or needle extends in a cantilevered fashion beyond thecorresponding substrate 36 as shown in FIG. 4. In the particularembodiment shown, the substrate or blade 36 is generally L-shaped inprofile and is edge-mounted on the dielectric board 26 so that the shortarm of each L-shaped blade extends through an opening 42 in the boardthereby allowing the needles to terminate below the opening. Asindicated above, blades having a construction of the type just describedare commercially available from Cerprobe Corporation of Tempe, Ariz.

Referring to FIG. 3, a plurality of inner conductive areas 44 are formedon the dielectric board 26 about the opening 42 in circumferentiallyspaced relationship to each other. Also formed on the board is an outerconductive area 46 which surrounds the inner conductive areas in spacedrelationship thereto. This outer conductive area extends outwardly tothe four edges of the board. As shown in FIGS. 3-4, a solder connection48 electrically connects the backplane conductive surface 40 of eachceramic blade 36 to a corresponding one of the inner conductive areas 44so that the ceramic blades are edge-mounted in radial arrangement-aboutthe opening 42. The ceramic blades are now prepared for connection withthe on-board portions 30 of the cables as will now be described.

FIG. 6 shows a transverse sectional view through the on-board portion3.0 of one of the cables 24 as taken along lines 6-6 in FIG. 1. Thisportion, which is of coaxial construction, includes an inner conductoror core 50, an inner dielectric 52, an inner layer 54, an outerconductor 56 and an insulative jacket 58. The inner layer 54 is ofsuitable composition for reducing triboelectric current generationbetween the inner dielectric and the outer conductor to less than thatwhich would occur were the inner dielectric and the outer conductor todirectly adjoin each other. As indicated in the Summary sectionhereinabove, this inner layer 54 should have physical properties similarto that of the inner dielectric 52 so that it does not rub excessivelyagainst the inner dielectric despite cable flexing or temperaturechanges. At the same time, this inner layer should have sufficientconductive properties to dissipate any charge imbalances that may arisedue to any free electrons that have rubbed of f the outer conductor. Asuitable material for this purpose is a fluoropolymer such as TEFLON™ orother insulative material such as polyvinylchloride or polyethylene incombination with graphite or other sufficiently conductive additive.

In the field of radio frequency (rf) cable technology, cables thatinclude a layer of the type just described are generally referred to as“low-noise” cables Commercial sources for this type of cable includeBelden wire and Cable Company based in Richmond, Indiana and SuhnerHF-Kabel based in Herisau, Switzerland. With regard to the preferredembodiment depicted, the cable which was used was purchased from TimesMicrowave Systems based in Wallingford, Conn. In order to provide thedesired twenty-four channel capability, these cables were custom orderedso that their diameter did not exceed that of standard RG178 cable.

It should be noted that some care must be exercised while connecting theon-board portion 30 of each cable to the corresponding probing device 34in order to prevent defects that would substantially degrade thelow-current measuring capability of the probe card. Referring to FIGS.3-4, a solder connection 60 connects the inner conductor 50 of eachcable to the rear end of the elongate conductive path 38 of thecorresponding probing device 34. Before making this connection, it isdesirable to position the cable so that the conductive and dielectriclayers in the cable that surround the inner core 50 are set back acertain distance 62 away from the rear edge of the probing device 34.This reduces the possibility that a fine strand of hair or is othercontaminant will form a low-resistance or conductive bridge so as tocause a low-resistance shunt or short across the signal line. Also, inmaking this connection, it is important not to overheat the cable so asnot to impair the structural properties of the material which forms theinner dielectric 52, which material can comprise, for example,air-expanded TEFLON™ for maximum temperature stability. Finally, afterthe connection has been made, all solder flux residue that remainsshould be removed from the board in order to prevent undesiredelectrochemical effects and to maintain the surface resistivity of theglass-epoxy board 26 at a reasonable level.

In order to further reduce the possibility of undesirable shuntingconnections, the outer conductor or metallic braid 56 of the cable isconnected indirectly to the backplane conductive surface 40 through thecorresponding inner conductive area 44, that is, a solder connection 64electrically connects the metallic braid to the inner conductive areaand a second solder connection 48 electrically connects the innerconductive area to the backplane conductive surface 40. Again, care mustbe taken not to overheat the cable or to leave solder flux residue onthe circuit board. During use of the probe card, the signal variation orvoltage is transmitted along the card by means of the inner conductor50, the elongate conductive path 38 and the probing element 22.Preferably, the test equipment is connected so that a feedback circuitin the output channel of the test equipment supplies a “guard” voltagethat matches the instantaneous signal voltage, which guard voltage isapplied to the outer conductor 56 and to the corresponding innerconductive area 44. In this manner, then, each elongate conductive pathis guarded by the backplane conductive surface 40 on the opposite sideof the blade 36 and by the corresponding inner conductive area 44 whichis arranged below the path. By minimizing leakage currents into and outof each elongate path, this guarding system reduces the levels ofundesired background current and so enhances the effect achieved inrespect to the cables due to the suppression of the triboelectriceffect.

Another potential source of current disturbance can arise due to acfields in the external environment surrounding the probe card. Oneconventional solution to this problem is to place the test instrument,interconnecting cables and probe card all together in a “wire cage” inorder to shield these components against ac pickup.

With respect to the exemplary probe card 20, the problem of ac pickup isaddressed in a more straight-forward fashion. FIG. 5 shows a transversecross sectional view of the lead-in portion 28 of a particular one ofthe cables 24 as taken along lines 5-5 in FIG. 1. In similarity with theon-board portion 30 of the cable as shown in FIG. 6, the lead-in portionincludes an inner conductor or core 50 a, an inner dielectric 52 a, aninner layer 54 a, an outer conductor 56 a and an insulative jacket 58 a.However, the lead-in portion of the cable further includes a secondinner dielectric 66 and a second outer conductor 68. In effect, thelead-in portion of the cable is a triaxial cable of “low-noise” typewhere the second outer conductor can be so connected as to serve as ashield conductor for self-shielding of the lead-in portions of thecables.

The second outer or shield conductor 68 is electrically connected to theouter conductive area 46 of the dielectric board 26 through the plug-intype connector 32, which connector has a metallized outer surfacesuitable for making such connection. The outer conductive area 46 iselectrically connected, in turn, to a conductive cover 70. When thesecond outer conductor is set to the shielding potential by the testinstrument, which potential is typically at ground, not only the lead-inportions of the cables but also the on-board probe card components aresubstantially shielded against ac fields In particular, the outerconductive area 46 and the conductive cover 70 form a self-contained andcompact shielding box that substantially surrounds and shields the probecard components that are mounted on the dielectric board Formed into thetop of the conductive cover is a viewing aperture 72 into which theviewing tube of a microscope can be inserted to facilitate viewing ofthe probing elements or needles 22 as they are being positioned on thecontact sites of the device under test.

It will be recognized that alternative forms of the invention arepossible without departing from the broader principles of the presentinvention. For example, if it is desired to handle a greater number ofchannels with the probe card, it is possible to modify the probe card toaccept a greater number of cables by using a cable connector in whichthe cables are mounted along two ranks instead of just one. Other designvariations will be evident to those of ordinary skill in the art.

The terms and expressions which have been employed in the foregoingspecification are used therein as terms of description and not oflimitation, and there is no intention, in the use of such terms andexpressions, of excluding equivalents of the features shown anddescribed or portions thereof, it being recognized that the scope of theinvention is defined and limited only by the claims which follow.

1. A probe card for probing a test device comprising: (a) a dielectricboard forming an opening; (b) a plurality of probing devices for probinga corresponding plurality of probing sites on said test device, eachprobing device including a dielectric substrate having first and secondsides, an elongate conductive path on said first side and an elongateprobing element connected to one end of said elongate conductive path soas to extend in a cantilevered manner beyond said substrate, saidprobing devices being edge-mounted in radial arrangement about saidopening so that said probing elements terminate below said opening in apattern suitable for probing said sites; and (c) a plurality of cablesfor connecting each probing device to a corresponding channel of a testinstrument, each cable including an inner conductor, an inner dielectricand an outer conductor, each inner conductor being electricallyconnected to a corresponding one of said conductive paths, each cablefurther including an inner layer of material between said innerdielectric and said outer conductor of suitable composition for reducingtriboelectric current generation between said inner dielectric and saidouter conductor to less than that which would occur were said innerdielectric and said outer conductor to directly adjoin each other. 2.The probe card of claim 1 wherein each probing device includes aconductive surface on said second side, each outer conductor iselectrically connected to a corresponding one of said conductivesurfaces, and said probing devices are arranged so that said conductivesurfaces alternate with said conductive paths in a circumferentialsequence.
 3. The probe card of claim 2 wherein said dielectric boardincludes a plurality of inner conductive areas surrounding said openingin circumferentially spaced relationship to each other, and eachconductive surface is electrically connected to a corresponding one ofsaid inner conductive areas.
 4. The probe card of claim 1 furtherincluding a conductive cover positioned over said dielectric board. 5.The probe card of claim 4 wherein said dielectric board includes anouter conductive area surrounding a plurality of inner conductive areasin spaced relationship thereto, and said cover includes a lower edgealong which said cover is connected electrically to said outerconductive area.
 6. The probe card of claim 4 wherein each cableincludes a lead-in portion leading to said board, said lead-in portionincluding a shielding conductor surrounding said outer conductor inradially spaced relationship thereto, said shielding conductors beingconnected electrically to said cover.
 7. The probe card of claim 6further including a cable connector through which said cables passdirectly into a shielded enclosure formed by said cover, said cableconnector providing an electrical connection path interconnecting saidshielding conductors and said cover.
 8. The probe card of claim 1wherein said dielectric board includes a plurality of inner conductiveareas surrounding said opening in circumferentially spaced relationshipto each other, each probing device being mounted so that each elongateconductive path is positioned above a corresponding one of said innerconductive areas, each outer conductor being directly connected to acorresponding one of said inner conductive areas.
 9. The probe card ofclaim 1 wherein said dielectric board is principally composed ofglass-epoxy material.